The general aims of this research are first, to investigate the mechanism(s) of forming chromosomal aberrations; second, to gain an understanding of the factors involved in the regulation of the number of tandem repetitions of a genomic segment in microorganisms; and third, to study, the process of bacteriophage growth by the use of chromosomal aberrations. Our approach to these problems is a genetic and physical analysis of exceptional T4 phage particles that are found among the progeny of crosses in which the parental phages carry overlapping but complementing deletions. Our working hypothesis is the exceptional phage particles in question contain a tandem duplication of a portion of the T4 genome that includes the r11 region. Duplications undergo assymetrical pairing and exchange resulting in the formation of structurally normal and tandem triplication chromosomes. During the current year our primary objectives were to critically test our tandem duplication hypothesis and to develop satisfactory methods for genetically mapping duplicated segments. The isolation of viable phage particles, segregated from tandem duplications, which carry three overlapping deletions and which exhibit the behavior expected of tandem triplications was deemed a critical test of our working hypothesis. Mapping experiments show that the duplications are relatively small--ranging from two to five cistrons in length.